Switched wired broadcasting systems are known in which television programs are made available on respective bus bars at a central exchange, subscribers are connected to the central exchange by respective cables, and remote control switching devices are provided to enable each subscriber to connect his own cable to a selected bus bar so as to receive the program made available on that bus bar. An example of such a system is described in British Pat. No. 1,272,594. It is also known to use the cable network of such systems to carry telephone services as described in British Pat. No. 1,414,127. A further development of switched systems is described in British Pat. No. 1,306,367 which provides for television signals generated by the subscriber to be returned to the central exchange and applied to one of the normal program bus bars for distribution to any subscriber connected to that bus bar.
The above described systems are capable of supplying a wide range of services and are inherently more flexible than systems in which all signals on the system are transmitted via a single coaxial cable of limited channel capacity. One desirable service that is not however provided is that of a viewphone, i.e. a two-way service capable of visually as well as audibly interconnecting two subscribers.
With the system of British Pat. No. 1,414,127, it is possible for a subscriber to apply a program signal to one of the system bus bars and then call by telephone another subscriber to instruct him to select that bus bar so that there is one-way visual communication. The calling subscriber cannot see the called subscriber as the called subscriber cannot return a program signal to the exchange as his equipment is occupied with receiving the signals originating with the calling subscriber. If the calling and called subscribers were to generate signals at different frequencies, the two subscriber generated signals could be applied to one channel without interference, i.e. one subscriber could select a particular bus bar, transmit a signal at a first frequency, and call a second subscriber by telephone to instruct him to select the same bus bar and transmit at a second frequency. There would then be two-way audio and visual communication. Unfortunately each subscriber to such a system would require a switchable two frequency receiving television set and a switchable two frequency transmitter for originating television signals. In addition the two subscribers would need to correctly tune the various system components prior to establishing visual communication. Thus not only would the system operator be required to supply relatively complex and therefore expensive equipment, but in addition the system subscribers would have to follow relatively complex and tedious procedures to make desired viewphone calls.
It is an object of the present invention to provide a broadcasting system which is capable of providing high quality television distribution and viewphone services, and which provides these services in a way which enables both services to make full use of the basic distribution network.
It has surprisingly been found that these objects can be met by providing for each subscriber to receive signals at a first frequency and by providing for each subscriber to transmit signals at a second frequency, two-way frequency changes being provided at a central location for converting subscriber-generated transmitted signals at the first frequency to the second frequency for reception by the other subscriber or subscribers.